"He's a dear, dear friend of a good friend of mine and I'm hesitant to say anything, in case it's the wrong thing. I mean I have nothing but nice things to say about him. I don't know whether I should talk about that situation. I don't feel comfortable."

Stacey, who starred with George Clooney and Catherine Zeta-Jones in the 2003 movie Intolerable Cruelty, said the encounter took place when John Major was PM between 1990 and 1997.

At the time Cameron was policy advisor to Norman Lamont during the Black Wednesday crisis which left millions in mortgage misery.

He and wife Sam - who were teenage sweethearts - are expecting their third child in February. Stacey said: "One of his closest friends from childhood is a dear friend of mine.

"And when I came over to England to work, or for various things, I would stay in my friend's flat, and his flatmate was David."

She added: "David would always be off writing speeches and following Major around Scotland or whatever, so I would stay in his flat."

In a conversation with one of Cameron's aides before speaking to the Mirror, Stacey joked she might spill the beans about what happened.

She admitted: "I said I would be talking to the Mirror and I said, 'Shall I tell that story about David', and we laughed. He goes, 'Oh God. We've got enough things floating around to deal with'.